diy

it's tedious and for a part like this, pointless, but have you ever used the spreadsheet workbench to make parametric models?

Oh yeah. I love doing that. Earlier when I was grumbling about annotations, at least in the spreadsheets you can write a couple sentences about what a variable does. If I'm "engineering" something from scratch, I'll typically put important dimensions and clearances in a spreadsheet and reference them within sketches. You need to be extra diligent when designing your model that it will successfully recompute given a range of values though. In the case of reverse-engineering, like this model, you throw design intent out the window and just pray everything fits.

Datum planes (as well as datum axes and points) are VERY useful. They are defined independently of any geometry in the model, and form a solid foundation to build other features from (i.e. a plane to sketch on, or an axis to bore a hole through or revolve around). You can also reference them in sketches just like any other part geometry, but they will ALWAYS be there when you change a dimension and recompute, unlike an arbitrary edge or vertex of a body. They are the canonical method to avoid the "topological naming problem." Even in commercial CAD systems like Creo Parametric or SolidWorks, which are somewhat smarter than FreeCAD about recomputing your model after changes, you WILL have problems if you're building a house of cards out of ephemeral geometry references on top of ephemeral geometry references.

Probably the most useful case for a datum plane, aside from a surface to sketch on, is a reference to perform a mirror operation. If you need to mirror a feature across a plane which isn't your base XY/YZ/XZ plane, you create a datum plane and mirror across that. Same goes for a datum axis. If you need to do a revolve operation or a radial pattern around an arbitrary axis other than X, Y, or Z, you create a datum axis and use that. These datums can also be referenced during a pad/pocket operation, where instead of specifying a dimension, you can tell the operation to pad/pocket until it hits a plane (which can be arbitrary and does not need to be parallel to the plane you are padding/pocketing from).

I've never used a multi-transform before, but all it does is combine several mirror/radial pattern/linear pattern operations into one operation. I do this to mirror features like the triangular ribs, then duplicate them at a linear offset. All a linear pattern does is duplicate geometry and translate it along an axis (i.e. turn one hole into a line of three holes). Apparently FreeCAD does not allow you to do a linear pattern of a mirror, or a mirror of a linear pattern, so these two operations need to be combined. I feel like I didn't run into this problem when I was using the RealThunder fork.